Track delivery type on connections.

Just round-robin in this commit. Will shortly expand to feature broadcast.
Use PUB-SUB messaging for broadcast delivery.

If we mark a connection as :delivery => 'broadcast', messages will be
delivered to EVERY subscriber (that is, all copies of a shard!) using
pub-sub messaging, instead of round-robining between subscribers.
(The default setting is 'round-robin'.)

Closes #41.
Merge pull request #43 from redjack/broadcast-messaging

Broadcast messaging